The present invention relates to article carriers and particularly to article carriers suitable for holding a group of metal cans and to the polymer composition from which the articles are formed.
In the past, several varieties of carriers have been used to contain metal cans in six packs and other arrangements. Such plastic web carriers are fabricated from a low density polyethylene (LDPE) resin material cut from a continuous plastic extruded sheet. The carriers have a unitary web main structure which has a plurality of can supporting and engaging loops or aperture portions. Six packs of beer and soft drinks are packaged in such band-type unitary flexible plastic web carriers.
During the application of the carriers the cans are grouped in continuous arrangements. The plastic carriers are installed on the cans by packaging equipment which applies the carrier to the cans at a very high speed, often as high as 1500 cans per minute. During the high speed application of the carrier the engaging loop portions of the carrier are stretched, placed over the cans, and subsequently released so that the stretched loops contract and securely engage the cans.
It is important during the stretching of the carrier loop portions over the cans that the resin material not be necked down. If a plastic material "necks down" during stretching, then that material is unacceptable for use as a web-type article carrier material. The resin material which "necks down" is unacceptable partly because it creates an unattractive display of the cans and partly because there is loss of the desired mechanical properties of elasticity and strength of the carrier.
In addition, during the application of the carrier it is important that there is a quick build up of sufficient tension in the loop portion such that the plastic material quickly snaps back and engages the can. If a plastic material does not "snap back" quickly, the packaging equipment cannot operate at its maximum line speed.
It is also important that the resin of the carriers rapidly establish sufficient tension to prevent can release during or immediately after the carrier application process. Therefore, the plastic material must have good short term recoverable stress characteristics. Even if the plastic material of the carrier has sufficient tension to prevent can release during the application process, it is also important that the carriers not lose much tension after a period of time due to relaxation of the plastic material and that the cans not slip from the plastic carrier. Therefore, any plastic material utilized for such carriers must maintain good resistance to stress relaxation over a prolonged period of time.
In addition to the severe demands required of resins useful for web carriers it is also desirable that the resin material be capable of being used with existing conventional processing and application equipment as well as being used with new and more efficient processing and application equipment. The resin material must maintain the desirable properties of the currently used web carriers.
The carriers should be able to withstand rotation or "facing" of the cans within the carrier during display in retail outlets. This rotation of the cans sometimes results in necking or breakage of the loops and the undesirable premature release of the cans. Also, it is desirable that the resin material of the carrier have sufficient stiffness so that the resin material does not flex during the production, storage or application process of the carrier. In addition, it is desirable that the carriers have sufficient stiffness so that a consumer may easily pick up the beverage package without having the carrier bend.
Also, the carriers should be resistent to various severe conditions during the application of the carrier on the cans. Sometimes these conditions cause environmental stress cracks in the carrier. Such conditions include exposure to grease and machine oil from the packaging equipment and cause the plastic material of the carrier to develop cracks to such a point that the plastic carrier becomes weakened, loses tension, and becomes more prone to tearing, thereby prematurely releasing the can from the carrier.
Due to competitive pressures, there is a desire to reduce the cost and improve the quality of the plastic web carriers. One possible way to reduce cost and improve quality would be to replace the LDPE resin material with a lower cost, higher quality resin material. The LDPE resin currently used to make the Hi Cone.RTM. (Hi Cone.RTM. is a registered trademark of Illinois Tool Works) plastic web carriers commands a premium price among the various grades of LDPE resins. The LDPE resin useful for a web carrier requires a unique combination of properties that are at the edge of technological and economical feasibility. Such LDPE resins require the unique properties of a low melt index, a relatively high density and an extremely broad molecular weight distribution. These requirements for the LDPE resins were established in order to to optimize stiffness and processability of the carriers. There are various grades of linear low density polyethylene (LLDPE) resin material which are stiffer, stronger and tougher than LDPE resins. The LLDPE resins also are generally more resistant to environmental stress cracking conditions than LDPE resins. The LLDPE resins are more cost attractive than LDPE resins because LLDPE resins are manufactured using a low pressure process which is less costly than the high pressure process used to manufacture LDPE resins.
However, there are several difficulties and drawbacks to using LLDPE resins in place of LDPE resins for use in making web carriers. The LLDPE resins have shorter molecular branches and a lower entanglement density which allows the molecules to "slip" past each other more easily. This "slipping" causes the tension in the LLDPE resins to relax over a period of time. The less "rubbery" characteristics associated with linear versus long branched polyethylene molecules interfere with the severe demands put on the resin material during the application process, handling and storage of the carriers. These severe demands on the web carriers include absence of "necking down" of the resin during stretching, rapid elastic recovery or snap back during carrier application to the cans, establishment of sufficient tension to prevent can release during application, and long-term can retention in the carrier during handling and storage. Also, LLDPE resins are known to be difficult to cut, thereby causing problems of achieving a clean cut during a punching press operations. Excessive scrap material of the LLDPE resin is generated and frequent cleaning of the punching press is required.
Until the present invention, carriers were not made using linear low density polyethylene resin materials. One obstacle in using a LLDPE resin is that LLDPE resins have a greater tendency to neck down during stretching than LDPE resins. The LLDPE resins have inferior elastic recovery characteristics such that during a high speed article carrier application process it would be expected that the loop portions would fail to rapidly snap back and maintain a firm tension grip against the cans. Also, LLDPE resins have been reported to show less resistance to creep and to stress relaxation. Creeping (or stretching of the material) occurs when the resin material is held under a constant force and the material stretches under such constant force. Stress relaxation occurs when the resin material is elongated and held constant at a predetermined elongated length for a period of time, during which time the force to maintain that elongated length decreases. Further, it would be expected that excessive generation of scrap material would occur during the punch press operations since the LLDPE resin material does not cut cleanly, and the stamping presses would need frequent cleaning.
Accordingly, there is a benefit to develop a web-type article carrier having advantageous cost, chemical and mechanical properties such that the use of such carrier increases manufacturing economics and finished product performance of the carrier. The carrier material should not neck down while being stretched during the application of the carrier on the cans. Further, the carrier material should snap back quickly to engage the can and should quickly build up sufficient tension such that the carrier material firmly engages the cans and allows the cans to be rapidly processed through the conventional application equipment. The carrier should continue to firmly engage the cans after application to the cans at least as well as present article carriers to insure can retention during packaging, storage, shipping to wholesalers and retailers, and final consumer use.
The carrier material should be capable of being formed into a variety of designs and be capable of gripping a variety of can surface finishes. The carrier material should resist degradation during packaging and processing of the packaged cans. The carrier also should have sufficient stiffness so that there is consumer ease of handling of the article carrier.